Today home appliances, OA equipment, electrical fixtures, etc., that are even higher performance and smaller and lighter weight are in demand and designs for maximizing the performance-to-weight ratio of an entire magnetic circuit that uses permanent magnets are being studied. A permanent magnet with a residual magnetic flux density Br of 5 kG to 7 kG is ideal for direct-current motors with a brush attached, which account for more than half [of the motors] produced today, but these cannot be obtained by conventional hard ferrite magnets.
For instance, the abovementioned magnetic properties are satisfied with Nd--Fe--B sintered magnets and Nd--Fe--B bonded magnets that are mainly Nd.sub.2 Fe.sub.14 B. However, Nd--Fe--B magnets contain 10 to 15 at % Nd, which requires many processes and a large facility for separation and purification and reduction of the metal, and therefore, when compared to hard ferrite magnets, they are very expensive. Consequently, these magnets have been promoted as a substitute for hard ferrite magnets only in some types of equipment because of the performance-to-cost ratio. An inexpensive permanent magnet with a Br of 5 kG or higher has yet to be discovered.
Moreover, a thin-plate permanent magnet wherein thickness of the permanent magnet itself is 100 .mu.m to 500 .mu.m is needed in order to realize miniature and thin magnetic circuits. Since it is difficult to obtain a bulk material of 500 .mu.m or less with Nd--Fe--B sintered magnets, thin-plate magnets can only be made by the method whereby sintered plates with a thickness of several mm are ground, or bulk material is sliced with a wire saw, etc., and therefore, there are problems in that finishing cost is high and the yield is low.
Nd--Fe--B bonded magnets are obtained by bonding powder with a thickness of approximately 30 .mu.m and diameter of several 10 .mu.m to 500 .mu.m with resin and therefore, it is difficult to mold bonded magnets where the thin-plate thickness is 100 .mu.m to 300 .mu.m.
On the other hand, an Nd--Fe--B permanent magnet whose main phase is an Fe.sub.3 B compound with an Nd.sub.4 Fe.sub.77 B.sub.19 (at %) neighboring composition has recently been presented (R. Coehoorn et al., J. de Phys, C8, 1988, pages 669-670), and the details of this technology are disclosed in U.S. Pat. No. 4,935,074, etc.
Moreover, prior to this, Koon presented a method of producing permanent magnets consisting of fine crystals by performing crystallization heat treatment on an La--R--B--Fe amorphous alloy comprising La as the essential element in U.S. Pat. No. 4,402,770.
It recently has been reported that thin pieces with hard magnetic properties are obtained by spraying Nd--Fe--B--V--Si alloy melt containing 3.8 at % to 3.9 at % Nd onto a Cu roller that is rotating to make amorphous flakes and then heat treating these at 700.degree. C., as disclosed in EP Patent Application 558691B1 by Richter et al. These permanent magnetic materials are obtained by crystallization heat treatment of amorphous flakes with a thickness of 20 .mu.m to 60 .mu.m and have a metastable structure with a crystal aggregate structure that is a mixture of an Fe.sub.3 B phase with soft magnetism and an R.sub.2 Fe.sub.14 B phase with hard magnetism.
The abovementioned permanent magnetic material has a Br of 10 kG and an iHc of 2 kOe.about.3 kOe and has a low content of Nd, which is expensive, of 4 at % and therefore, the starting material cost is less expensive than with Nd--Fe--B magnets whose main phase is Nd.sub.2 Fe.sub.14 B. However, there are limits to the liquid solidification conditions, which are essential to making an amorphous alloy from the starting mixture, and, at the same time, there are limits to the heat treatment conditions for obtaining a material with hard magnetism. Therefore, [such magnets] are impractical in terms of industrial production and as a result, there is a problem in that they cannot be inexpensively presented as a substitute for hard ferrite magnets. Moreover, said permanent magnet materials are obtained by crystallization heat treatment of amorphous flakes with a thickness of 20 .mu.m to 60 .mu.m, and therefore, permanent magnets having a thickness of 70 .mu.m to 500 .mu.m as required for thin-plate magnets cannot be obtained.
On the other hand, U.S. Pat. No. 508,266, etc., disclose the fact that rapidly Nd--Fe--B magnetic materials consisting of a structure consisting of crystals with hard magnetic properties are directly obtained by rapidly solidifying an alloy melt on a roller at a circumferential speed of 20 m/s. However, since flake thickness of the rapidly solidified alloy obtained under these conditions is thin at approximately 30 .mu.m, they can be crushed to a powder particle diameter of 10 .mu.m to 500 .mu.m and used as the abovementioned bonded magnets, but they cannot be used for thin-plate magnets.